Neurons immunoreactive to luteinizing hormone-releasing hormone are present in the medial preoptico-septal region of the rat hypothalamus, an area known to be essential for mediating the surge of secretion of the luteinizing hormone (LH) from the anterior pituitary that precedes ovulation. In preliminary studies it has been shown that the stereotaxic microinjection of neurotensin (NT) into a discrete area of the rostral, medial preoptic nucleuis (MPO) results in an elevation of plasma LH levels. It is the purpose of this study to further investigate the action of NT into the rostral MPO of conscious, cycling rats while assaying plasma LH levels, (2) the direct micro-injection of a newly developed ligand which binds irreversibly to NT receptors to determine the functional role of endogenous NT on LH surge and ovulation, and, (3) the direct microinjection of NT in the central regulation of LH release by: (1) the direct microinjection of NT into the rostral MPO following the previous micro injection of drugs which deplete or block the effect of catecholamines to determine whether the action of NT is mediated through a catecholaminergic pathway. All of these studies will be performed on conscious, cycling rats using site specific micro injections. Hopefully these studies will generate more information about the CNS regulation of LH secretion and will enhance our understanding of reproductive physiology and may aid in the evaluation of reproductive disorders of central origin.